On my heating oil tanks I have 'Watchman' monitors, that radio on an infrequent basis to an indoor receiver to tell me how much oil is left. I have three separate heating systems in three buildings - they all have Watchman monitors of varying vintages, but all have one thing in common. A 6 volt battery encased in a copper tube entirely proprietary and hugely expensive.

I recently took delivery of a 'top up' of oil (2300 litres and that's a top up ) after which the monitor for my workshop ceased to work. Obvious first thing to look at is the transmitter battery - sure enough it's down to 5 volts off load so change the battery. New genuine battery over £30 !

Now I know from previous investigations that these things consist of a screw connector sealed in the end of a piece of standard 15 mm copper plumbing pipe, which has a cap at the far end, and a plastic sleeve bringing the bore down to the diameter of an AAA battery. Inside are four AAA batteries and a spring to make contact.

So - mount the pipe in the lathe - carefully part it off down to the plastic liner and clean the ends. Then take a 15 mm compression joiner - bore out the centre stop so that the pipe will slide all the way back together, and re-assemble with new batteries and lashings of Vaseline as that is what it seems to have been previously smeared in.

Actually slightly more complicated - I only had a bulk head fitting with no conical ends for the 'olives' so I had to machine them.

Wack it back on the transmitter and wait to see if now it will talk to the receiver. This can take a long time so the fault may or may not be fixed - but the battery is at least now good

Biggles, only if ohms law has been revoked - at a given voltage it will draw what it wants to draw

Howsitwork - in a word no I've withdrawn the entire gizmo from the tank and got it in the workshop adjacent to the receiver. It works by firing a sound burst down a tube vertically immersed in the oil. It listens for the echo off the surface of the oil and calculates the oil level. Googling it, it has a 'calibration cycle' to go through when it's been powered off for more than two minutes. It fires a few sound bursts for 25 seconds then averages the delay to receiving the echo. After 25 seconds it's supposed to emit three bleeps then it's calibrated. Setting it off you can hear the sound bursts being emitted, but they carry on for several minutes then cease. I assume that the sound receiver has failed and it's not hearing the echos - replacement on order !

I have the same watchman monitor on my LPG bulk storage tank. The service engineer who fitted it told me that the copper cap on the end of the tube comes off with a little persuasion with two pairs of grips. He advised me to just swap the AA cells out rather than buy replacement tubes which the company he works for happens to sell to their customers .